1. Field of the Invention
The disclosed invention relates to a storage device.
2. Description of the Related Art
Signal processing units such as central processing units (CPUs) vary in structure depending on the intended use. A signal processing unit generally has a main memory for storing data or program and other memory circuits such as a register and a cache memory. A register has a function of temporarily holding a data signal when arithmetic processing is carried out, when a program execution state is held, or the like. Meanwhile, a cache memory, which is located between an arithmetic unit and a main memory, is provided to reduce low-speed access to the main memory and speed up arithmetic processing.
In a memory circuit in a signal processing unit, such as a register or a cache memory, input of a data signal needs to be performed at higher speed than in a main memory. Thus, in general, a flip-flop or the like is used as a register, and a static random access memory (SRAM) or the like is used as a cache memory. In other words, such a register, a cache memory, or the like is a volatile memory circuit which loses a data signal after the supply of power supply potential is stopped.
In order to reduce power consumption, a method has been suggested in which the application of source voltage to a signal processing unit is temporarily stopped while input/output of data signals is not conducted (see Patent Document 1, for example). In the method in Patent Document 1, a nonvolatile memory circuit is located on the periphery of a volatile memory circuit such as a register or a cache memory, and the data signal is temporarily stored in the nonvolatile memory circuit. Thus, in the signal processing unit, the data signal stored in the register, the cache memory, or the like can be held even while the supply of source voltage is stopped.
In the case where the application of source voltage to a signal processing unit is stopped for a long time, a data signal in a volatile memory circuit is transferred to an external memory circuit such as a hard disk or a flash memory before the application of source voltage is stopped, so that the data signal can be prevented from being lost.